Page 145 - Design of Solar Thermal Power Plants

130 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

1. Molecule absorption. Atmospheric molecules polarize under the
influence of the optical wave electric field and perform forced
vibrations on the frequency of incident radiation. Thus in order to
overcome the internal resistance of atmospheric molecules, energy is
consumed and takes the form of absorption of atmospheric
molecules.
Molecular absorption characteristics strongly rely on the frequency
of radiation wavelength. The inherent absorption frequency of a
molecule is determined by its internal motion modality. The internal
motion of a polar molecule normally consists of the electronic motion
within the molecule, vibration of atoms that compose the molecule,
and rotation of molecules surrounding the center of mass. The
respective resonant absorption frequencies correspond to the
ultraviolet, visible light, near-infrared, intermediate-infrared, and
far-infrared regions of the optical wave.
Although N 2 and O 2 molecules take up the largest share of the
atmosphere (about 90%), their visible light and infrared regions
show hardly any absorption, and then only show significant
absorption toward far-infrared and microwave bands. Thus within
the visible light and near-infrared regions, the effects of absorption
are normally not considered.
Except for the above molecules, the atmosphere also contains He,
Ar, Xe, O 3 , Ne, and so on. All these molecules have considerable
absorption spectral lines in the visible light and near-infrared
regions; however, due to their insignificant amount in the
atmosphere, the respective effect of absorption is normally also not
considered. Only when the remaining attenuation factors are
already quite weak above the sky is the absorption effect considered.
For a tower power plant, the influences of molecular absorption are
not considered.
H 2 O and CO 2 molecules also play a certain role in the absorption
of solar radiation. In special cases, the H 2 O molecule has a broad
vibrationerotation and sole-rotation structure within the near-
infrared region, and thus the H 2 O molecule acts as the most
important absorption molecule in visible light and near-infrared
regions and serves as the main cause for optical attenuation of the
atmosphere on clear days. The central wavelengths of some major
absorption spectral lines of H 2 O molecules are as follows: 0.72, 0.82,
0.93, 0.94, 1.13, 1.38, 1.46, 1.87, 2.66, 3.15, 6.26, 11.7, 12.6, 13.5, and
14.3 mm.
For certain specific wavelengths, the atmosphere may present
extremely intensive absorptions that optical wave can barely pass
through. Solar radiation wavelength bands applied by CSP
generation are mainly below 3 mm and greatly overlap with the